1. Field of the Invention
The present application relates to an optical transceiver, in particular, the present application relates to an optical transceiver capable of performing the coherent optical communication.
2. Background Arts
As the mass of information to be transmitted on the optical communication system, endeavors have been continuously devoted to enhance the transmission capacity not only to increase the transmission speed but to raise the modulation degree, such as the polarization modulation, the phase modulation, a combination of these two modulations, and so on. These modulations have been known as the coherent modulation technique in the wireless communication. Recently, various documents have reported to introduce the coherent modulation in the optical communication system.
The coherent modulation counts the phase of the light as one information unit. Accordingly, comparing the phase of the transmitted light with the reference light, the in-phase component and the quadrature component may be utilized as the information unit. The former (In-phase component) is a component whose phase matches with that of the reference light, while, the latter is a component whose phase is difference by 90° against that of the reference light. In the coherent modulation, the reference light is called as the local light whose frequency is precisely matched with that of the signal light, while, the phase is optional against the signal light. The optical communication system may utilize, in addition to those two components in the phase, two polarizations may be utilized as the information unit. Such a modulation is called as Dual-Polarization Dual-Phase-Shift-Keying (DP-QPSK).
The DP-QPSK modulation requests the local light to be extremely stable in the phase thereof, namely, extremely narrow linewidth of the laser emission, and to be compact as possible. In order to get the narrow line width, the optical modulation is carried out by an optical modulator independent of the LD, in particular, the coherent modulation generally introduces, what is called, the Mach-Zehender (MZ) modulator. When the MZ modulator is primarily made of dielectric material, typically lithium niobate (LN), and a substantial longitudinal length is inevitable to secure an effective electrical-to-optical interaction because of relatively smaller electro-optic effect of the material.
Most optical transceivers distributing in the field are defined in the outer dimensions and electrical interfaces thereof in respective standards. Because the optical transceivers are operated under controls by the host system, diverse outer dimensions and electrical interfaces delay the diffusion of such optical transceivers. Accordingly, a coherent optical transceiver is necessary to install a lot of optical components, compared with conventional optical transceivers adopting only the amplitude modulation, within a housing having a limited inner space.